In order to obtain an optical data recording medium having bigger storage capacity, there has been a demand for a technique to record and reproduce a signal in accordance with a further shorter mark length (pit length) in the optical data recording medium. Recently, an optical data recording medium has been developed which enables reproduction of a signal from a mark having a mark length shorter than a mark length of resolution limit of an optical system of a reproducing apparatus. Hereinafter, the optical data recording medium is referred as a “super-resolution optical data recording medium.”
The super-resolution optical data recording medium includes at least a reproducing layer and a reflective layer layered in this order on a substrate. The super-resolution optical data recording medium employs a method of virtually reducing a spot size of a laser beam which reaches the reflective layer. Specifically, the spot size of the laser beam which reaches the reflective layer is reduced by irradiating a reproducing laser beam via the substrate to the reproducing layer. In this way, the reproducing layer has such an optical characteristic with respect to the reproducing laser beam that is distributed unevenly over the reproducing layer.
In other words, light intensity in the spot of the reproducing laser beam irradiated to the reproducing layer is unevenly distributed. Because of this, temperature is also distributed unevenly therein spontaneously. Therefore, in such an arrangement wherein the reproducing layer is made of a material whose optical characteristics (mainly transmittance) is are changed by temperature or light intensity distribution, it is possible to increase only the transmittance of that part of the reproducing layer which is at the center of the laser beam spot because temperature is high and light intensity is high in the part at the center of the laser beam spot. When transmittance is increased only in that part of the reproducing layer which is at the center of the laser beam spot, the reflection layer receives only the light of the center of the laser spot. That is, the laser beam spot irradiated on the surface of the reflective layer is virtually reduced. Therefore, in the super-resolution optical data recording medium, it is possible to reproduce a mark having a mark length shorter than a mark of resolution limit of the optical system.
As an example of a layer material whose optical characteristic is changeable by light intensity, a shutter layer (a layer in which semi-conductor fine particles are dispersed in a matrix made of glass or resin) is described in Japanese Publication for Unexamined Patent Application No. 6-28713 (Tokukaihei 6-28713, published on Feb. 1, 1994). In the arrangement of the patent application, as shown in FIG. 7, the shutter layer (reproducing layer) 42 and the optical reflective layer 44 are layered on that surface of the substrate 45 which is reverse to the surface from above which the laser beam is irradiated.
Incidentally, as an example of a raw material whose transmittance is increased by high temperature, a thermochromic pigment is described in Japanese Publication for Unexamined Patent Application No. 2001-35012 (Tokukai 2001-35012, published on Feb. 9, 2001). In the optical data recording medium described in the patent application, as shown in FIG. 8, a mask layer (reproducing layer) 32, a first dielectric layer 36, a phase change recording film 37, a second dielectric 38, a reflective layer 34, and a protective resin layer 39 are layered in this order on that surface of the substrate 35 which is reverse to the surface from above which the laser beam 30 is irradiated.
As described above, in each conventional super-resolution optical data recording medium, the reproducing layer is provided on that surface (non-light-incident surface) of the substrates which is reverse to the surface (incident surface) from above which the laser beam is irradiated.
However, in the conventional optical data recording media, resolution limit is not enough. Therefore, there is a demand for an optical data recording medium having a greater resolution limit.
In the arrangement in which a reproducing layer is provided on the non-light-incident surface, the reproducing layer cannot be thicker because a recording layer and a reflective layer are also provided in the optical data recording medium. Accordingly, the optical data recording medium including a further shorter mark length cannot be reproduced.
Specifically, in the conventional optical data recording medium, generally the reflective layer has a non-flat surface (a rise and/or a recess, for example formed by pits and/or groove, or the like). When the laser beam is irradiated to the non-flat surface, a laser beam reflected from the rise part of the non-flat surface is different in quantity from one reflected from the recess part of the non-flat surface because of interference. By using the difference, tracking on grooves, and signal reproduction are performed. The rise and/or the recess of the reflective layer is formed by forming a non-flat surface on the substrate by using a pit and a groove, or the like, which are for storing data or for locating a reproducing point. The reflective layer is layered on the substrate. Thereafter, in the conventional arrangement, the reproducing layer is provided on the substrate having the rise and/or the recess, and the recording layer and the reflective layer are layered in this order on the reproducing layer. Accordingly, when the reproducing layer is too thick, the rise and/or the recess are leveled off, and thus the recording layer and the reflective layer cannot have a non-flat surface.
In the above-mentioned Japanese Publication for Unexamined Patent Application No. 6-28713, there is an example that the rise and/or the recess of the substrate are leveled off because the reproducing layer layered on the substrate is too thick. In the example, a resin layer is used as the reproducing layer, and the resin is so adhesive that it is very difficult to attain a thin thickness of the resin layer. Thus, there is a high possibility that the formation of the resin layer will level off the non-flat surface of the substrate, the rise and/or the recess being a source of data. In case the reflective film is provided on the substrate having such leveled-off non-flat surface, no interference in the reflected light beam will be caused by the leveled-off rise and/or the leveled-off recess, whereby, data cannot be read out. Moreover, as to an inorganic film, which may have a thin thickness, the same is true that there is a possibility that a thick thickness of the inorganic film will level off the rise and/or the recess. Therefore, there is a limit in how thick the reproducing layer can be.
The arrangement in which the thickness of the reproducing layer is thin in view of the above limitation faces the following problem: for example, in the case that the reproducing layer has a greater transmittance with a thicker thickness, the thin thickness limits how much the laser spot can be reduced, thereby prohibiting the optical data recording medium from having a better resolution limit.
Also, it is considered that the resolution limit of the optical data recording medium is limited by various factors apart from the limitation of the thickness of the reproducing layer.